Limb lengthening devices known as distractors are used in the treatment of bone defects and injuries, and for lengthening bone in connection with osteotomy (for example, complete osteotomy or corticotomy).
A distractor has a length that is adjustable (e.g., expandable or telescoping). The distractor generally includes, proximate to each end of the length, one or more fixators for attaching the distractor to a bone in a limb of a subject (e.g., a patient). The distractor is typically applied by affixing each fixator to the bone (such as by using pins, screws, Kirschner wires, and the like), such that each end of the distractor is attached to the bone at an opposite side of a distraction gap created by the osteotomy. The distractor is able to elongate the distance between the fixators at each end of the distractor, thereby applying force or tension to the bone over a period of time, to gradually lengthen the bone in small increments. The bone is lengthened by osteogenesis (i.e., the formation of new bone), bridging the distraction gap. In a typical example, a treatment regime may call for a predetermined distraction rate of one millimeter per day (mm/day) during a treatment period.
The rhythm of distraction, i.e., the frequency of lengthening the distractor, is directly related to the speed of osteogenesis. Some distractors are configured to be lengthened periodically, e.g., by manually adjusting the length of the distractor a number of times each day during the treatment period, so as to increase the distraction by a given amount with each adjustment. Other distractors, generally known as automatic distractors or autodistractors, are configured with a motor that is able to continuously increase the length of the distractor at a predetermined distraction rate during the treatment period. Autodistractors that produce a continuous and gradual distraction (e.g., elongating the distractor at a predetermined distraction rate of about 1.0 mm/day) have been found to produce more rapid osteogenesis than distractors that are periodically lengthened (e.g., several times a day).
Distractors may be instrumented with a displacement sensor for determining actual distractor displacement; that is, the displacement sensor allows a practitioner (e.g., a physician, surgeon, clinician, or researcher) to monitor changes in the length of the distractor or the distance between the fixators. An autodistractor may also be instrumented with a force sensor (e.g., a strain gauge, or a load cell) for sensing the force applied by the distractor to the bone, and with a controller able to disable the motor if a predetermined threshold of force has been exceeded.
To achieve a desired elongation rate, autodistractors have applied a constant gain (i.e., constant voltage) to the motor, producing a constant force to elongate the bone. The desired rate of distraction is set by a practitioner, and this is used to determine the constant voltage applied to the motor. The actual rate of bone distraction may or may not mimic the desired rate of distraction, depending upon factors including resistant force encountered in the limb. The motor drives at its specified rate regardless of the resistant force it encounters. As a result, inappropriate forces may develop in the distraction zone. These forces could either be too high or too low. If high forces are allowed to build up, this could indicate that there is premature consolidation, or conversely, that there is too much separation resulting in high stresses on the surrounding soft tissue. In the case of low forces, this might indicate that there is insufficient bone formation in the distraction gap.